fix: security hardening and architectural improvements for custom menu

1. (Critical) Filter admin-only menu items from public API responses -
   both GetPublicSettings handler and GetPublicSettingsForInjection now
   exclude visibility=admin items, preventing unauthorized access to
   admin menu URLs.

2. (Medium) Validate JSON array structure in sanitizeCustomMenuItemsJSON -
   use json.Unmarshal into []json.RawMessage instead of json.Valid to
   reject non-array JSON values that would cause frontend runtime errors.

3. (Medium) Decouple router from business JSON parsing - move origin
   extraction logic from router.go to SettingService.GetFrameSrcOrigins,
   eliminating direct JSON parsing of custom_menu_items in the routing
   layer.

4. (Low) Restrict custom menu item ID charset to [a-zA-Z0-9_-] via
   regex validation, preventing route-breaking characters like / ? # or
   spaces.

5. (Low) Handle crypto/rand error in generateMenuItemID - return error
   instead of silently ignoring, preventing potential duplicate IDs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/dto/settings.go

@@ -169,3 +169,15 @@ func ParseCustomMenuItems(raw string) []CustomMenuItem {
 	}
 	return items
 }
+
+// ParseUserVisibleMenuItems parses custom menu items and filters out admin-only entries.
+func ParseUserVisibleMenuItems(raw string) []CustomMenuItem {
+	items := ParseCustomMenuItems(raw)
+	filtered := make([]CustomMenuItem, 0, len(items))
+	for _, item := range items {
+		if item.Visibility != "admin" {
+			filtered = append(filtered, item)
+		}
+	}
+	return filtered
+}